I saw an ad on TV the other day, promising that its product would "kill 99% of most household germs", or something like that. Of course, "most" could mean anything over 50%, and to take a proportion (even 99%) of that will make it even less, so the promise doesn't really say much. It's almost as bad as the sales where everything is "up to 50% off" (which of course is consistent with most of the discounts actually being far less). Damn advertisers.
More generally though, it can be difficult to report second-order percentages in a non-misleading way. Suppose a political candidate starts with 10% of the vote. If someone reported that his popularity had "increased by 50%", you might be startled into thinking that he was now winning the race, with 60% of the vote. But they might instead merely mean to say that he increased his vote share by five percentage points, to a total of 15% -- which is, after all, half again as good as (hence, 50% improvement on) his original position of 10%.
Anyway, it would be nice if people took more care to resolve this ambiguity when reporting these sorts of statistics. Clarity might be achieved by describing the first sort of increase as "an improvement of 50 percentage points", which I think is more clearly talking about the first-order units. To report second-order percentages, one might specify the background set, e.g. "50% of his original 10 per cent share".
Such care might significantly improve 99% of some of the world.